1. Field of Invention
This invention relates to the preparation of cyanuric acid and, in particular, relates to the conversion of urea to cyanuric acid.
2. Brief Statement of the Prior Art
Cyanuric acid is prepared commercially by thermal decomposition of urea. Upon heating, urea forms biuret and triuret which cyclizes with evolution of ammonia and water into cyanuric acid and cyclic by-products such as ammeline and ammelide. The cyclic by-products can be readily hydrolyzed to cyanuric acid via acid or base hydrolysis.
The efficiencies of the prior thermal techniques are greatly impaired by the physical properties of the crude reaction product which is a mixture of cyanuric acid and the cyclic by-products. The crude reaction product solidifies at the reaction temperature, typically up to about 300.degree. C., and the solidified mixture is an excellent thermal insulator. Increasing the temperature to improve the thermal heat transfer to the reaction mixture only results in depolymerization and degradation of the product.
Many techniques have been proposed to obviate or circumvent the problems presented by the physical nature of the reaction mixture. These techniques have included heating of urea in a particle form using a rotating kiln or a fluidized bed. Other techniques have suggested heating of urea on a molten bath of zinc, lead, or tin or dissolving the urea in an inert high boiling solvent such as propylene glycol. All of these techniques have some disadvantages; e.g., rotating kilns and fluidized beds present equipment and solids handling problems while solvents must be recovered from the reaction product.